For the purposes of the present discussion, the present invention will be discussed in terms of a “white” emitting light-emitting diode (LED); however, the methods taught in the present invention can be applied to wide range of LEDs. A white emitting LED that emits light that is perceived by a human observer to be “white” can be constructed by making an LED that emits a combination of blue and yellow light in the proper ratio of intensities. High intensity blue-emitting LEDs are known to the art. Yellow light can be generated from the blue light by converting some of the blue photons via an appropriate phosphor. In one design, a transparent layer containing dispersed particles of the phosphor covers an LED chip. The phosphor particles are dispersed in a potting material that surrounds the light-emitting surfaces of the blue LED. To obtain a white emitting LED, the thickness and uniformity of the dispersed phosphor particles must be tightly controlled.
In one prior art method for constructing such a device, the phosphor is mixed with a resin material such as epoxy or silicone and the slurry is put over the LED chip. The phosphors are typically in the form of fine particles and usually have a distribution typically ranging from 1 um to 20 um. When the slurry is used to cover the LED chip, the phosphor particles are initially distributed throughout the coating layer and occupy a volume greater than the LED chip.
Such devices have a number of problems. First, if the resin does not cure quickly, the phosphor particles tend to settle, and hence, there is a non-uniform distribution of particles that often has a boundary between the region of the resin having the particles and the upper portions of the resin coating. This boundary can cause the coating to split into two layers at some point in the life of the light source.
Even if the resin sets before the particles have time to settle, the resulting light source is a three-dimensional source of a size that is much larger than the underlying LED chip. Such a source presents problems in applications in which an optical system must be used to image the light source onto an object that is to be illuminated. The light source is essentially a compound light source having a first point source that emits blue light and a broader diffuse source that emits the yellow light. Consider an optical system that images this compound source onto a scene that is to be illuminated with white light. To be perceived as white light, each area of the scene must receive the same amount of blue and yellow light. Consider a collimating lens that has the LED at its focal point. The blue light will be formed into a beam having a more or less uniform intensity. The yellow light will, in general, not be uniformly distributed across this beam, since the yellow light source is not at the focal point of the lens and consists of a broad three-dimensional source. Hence, a human observer will see a source that varies in color across the source.